I used to see any .net source code (like System.String) with ReSharper and Visual Studio 2008. since I migrated to Visual studio 2010, this option doesn't give the full source code but just the definitions of the class.
How can I fix this, to use the microsoft's symbols server again?
Edit: Maybe the problem is only with .net-4.0 assemblies. is microsoft released symbols to to .net-4.0 assemblies also?
Microsoft released sources for .NET 4 release, so R# have to work just fine. Some debugging information may be found in View -> Output -> ReSharper after unsuccessful navigation. Sometimes R# can't download sources or symbols files because of server errors on referencesource.microsoft.com side.
If you go to ReSharper -> Options -> Tools -> External Sources, you can set the preference order of where ReSharper gets its definition view from. Make sure "Sources from symbol files" is checked, and check the options at the bottom of that page.
You can download all of the source and symbols and install it in one go.
and then follow adrian's advice.
来源：https://stackoverflow.com/questions/2925149/how-to-see-net-library-source-code-with-resharper-5-and-visual-studio-2010